The best combination of bore and stroke that I've found that comes the closest to your goal is a 69.5mm bore and a 76mm stroke for 288cc. The main problem with the YZ and RM is the cylinder head stud location and relatively small diameter of the inner o-ring. Relocating the o-ring is possible by welding the top of the cylinder and re-cutting the groove to a larger diameter in order to get it farther from the bore, but thats very difficult.
I use the Wiseco 69.5mm from a TRX 4-wheeler part # 526MO6950 then weld up the outer edges of the rear transfers to accomodate the ring centering pins. Then I have Crank Works off-set the crank pin holes by 2mm and press in a Hot Rod kit. The crankcases will also have to be machined to accomodate the connecting rod and a spacer plate 1mm with 2 standard gaskets must be used on the cylinder base. This set-up produces and engine with a strong pulling mid-range and good top end. I change the porting to suit the application and that may include a thinner spacer plate for more low end. This set-up is the most reliable that I've found. The 74mm piston that I designed when I worked at Maax Power can be adapted but its not very reliable. Some companies are using that piston with stroking to get it out to 327cc but theres only so much you can do with the original design. The last thing you want is a 250 that you have to rebuild once a month.
Good luck on your project, Eric